1. Field of the Invention
The present invention relates to a transmission and, more particularly, a device for controlling operation of a lock-up clutch incorporated in an automatic transmission system having a torque converter.
2. Description of the Prior Art
An automatic transmission including a fluid torque converter and a lock-up clutch is, of course, know in the prior art. U.S. Pat. No. 3,255,642 apparently discloses an automatic transmission including a fluid torque converter and a lock-up clutch. In a transmission including a fluid torque converter and a lock-up clutch, it is required that the lock-up clutch is disengaged not only when the transmission is shifted in a low speed range, but also when the transmission is changed over between two different speed ranges. In the aforementioned U.S. patent, the temporary disengagement of the lock-up clutch during change-over between two speed ranges is effected by employing a pressure difference caused by a restricting orifice through which servo oil pressure is transmitted to individual clutches and/or brakes which effect a different power transmission route in a gear transmission. However, a system which depends upon the pressure difference across a restricting orifice means does not operate consistently because the operating characteristic varies largely in accordance with the viscosity of oil, i.e. the temperature thereof. In view of such a problem in the earlier prior art, U.S. patent application Ser. No. 634,040 filed on Nov. 21, 1975, and assigned to the same assignee as the present application, was directed to employing a change-over valve composite adapted to be actuated by a plurality of control fluid pressures generated by a transmission control system so as to control the supply of a selected one of the control fluid pressures to an actuating port of a lock-up clutch control valve depending upon a balance among the plurality of control fluid pressures.